Polyethylene microplastics as emerging reproductive toxicants: Evidence from an oocyte-based mammalian model.

Microplastics (MPs) are emerging environmental contaminants detected in biological fluids, including blood and follicular fluid, raising concern about their potential effects on mammalian reproduction. The present study investigated whether polyethylene microplastics (PE-MPs; 10-30 μm), at concentrations reported in vivo, affect bovine oocyte maturation and subsequent embryonic development. Bovine cumulus-oocyte complexes were matured in vitro for 24 h in the presence of 0, 0.025, 0.5, or 1.5 μg/mL PE-MPs. Nuclear maturation, mitochondrial distribution, mitochondrial content, mitochondrial membrane potential (ΔΨm), intracellular glutathione (GSH) in oocytes, progesterone secretion by cumulus cells, and embryo development after in vitro fertilization were evaluated. PE-MPs exposure significantly impaired meiotic progression. Mitochondrial content decreased at all concentrations, accompanied by abnormal mitochondrial aggregation and altered distribution, particularly at intermediate concentration, whereas ΔΨm remained unchanged. Intracellular GSH levels were reduced at 0.5 and 1.5 μg/mL. In contrast, progesterone secretion and early embryo development were not affected. Nuclear maturation, mitochondrial aggregation and migration and GSH levels exhibited quadratic dose-response patterns, suggesting a non-monotonic response consistent with adaptive cellular stress rather than overt cytotoxicity. The combination of preserved bioenergetic activity with mitochondrial redistribution and oxidative imbalance indicates activation of mitochondrial quality-control mechanisms. These findings support an endocrine-disruptor-like mode of action affecting regulatory pathways controlling meiotic maturation rather than steroidogenic capacity. In conclusion, PE-MPs disrupt bovine oocyte maturation by targeting mitochondrial organization and redox homeostasis without compromising overall cellular viability or embryonic competence. The oocyte therefore represents a highly sensitive early indicator of reproductive toxicity.